Specifying ceilings and HVAC equipment to meet acoustic requirements

So, which ceiling panels perform the best? Every ceiling panel performs better in some respects and worse in others. Some panels do not prevent noise leaks along the grid flanges, but they limit the noise passing through the panel itself. Other panels absorb the reflected noise in the plenum and room better, and prevent noise leaks along the grid even though more sound passes through them. In conclusion, the ASHRAE Handbook HVAC Applications, when discussing the attenuation performance of various ceiling panels, states, “Experiments have shown that, for ceiling panels supported in a T-bar grid system, differences among ceiling panel types are small.”

The evidence

To better understand the claims, data, and prediction method in the ASHRAE Handbook and AHRI standard, one should refer to ASHRAE RP-755. This study was conducted by the National Research Council Canada’s (NRCC) Institute for Research in Construction (IRC) (currently the Construction Research Centre). The NRCC is Canada’s largest federal research and development organization.

The RP-755 study “was initiated to investigate the transmission of sound through different ceiling types with the intent of providing more reliable design information to deal with sound transmission through ceilings close to HVAC devices.”

Within a large, acoustic chamber, the NRCC researchers tested a wide variety of ceiling panels including those made of fibreglass, mineral fibre, and gypsum board. Ceiling panel thicknesses varied from 13 to 50 mm (0.5 to 2 in.). Weights varied from 0.5 to 8.9 kg/m2 (0.1 to 1.8 psf). CAC ratings varied from 28 to 39. Sound isolation ratings for ceiling type 3 were not measured. STC ratings varied from 16 to 19, while NRC ratings varied from 0 to 1.1 (refer to Table 1). These panels represent the full range of acoustic ceiling panels available in the market at the time of the study and still today.

Table 1 Compiled information for each ceiling type studied in ASHRAE RP-755, and reported in ASHRAE Handbook HVAC Applications, and AHRI Standard 885. 5

The NRCC researchers used various types of mechanical equipment above the ceilings as the noise sources. These devices were installed as they would be in a building, having air moving through them, and conditioning the air. For each ceiling type, and each piece of mechanical equipment, the researchers measured the resulting noise levels in the room below at occupant ear height. They compared the known sound power levels of the mechanical equipment to the measured noise level at listener height to derive the attenuation being provided by each ceiling system.

The study was empirical, it was not theoretical or based on calculations, models, or simulations. Figure 2 shows the average attenuation by frequency octave band of each ceiling across all the different types of mechanical devices used in the study.

Figure 2 Average noise attenuation (vertical axis) across different frequency octave bands (horizontal axis) for different acoustic ceiling panel types in a suspended grid when the noise sources are HVAC equipment in the plenum and located close to the ceiling. Refer to Table 1 for detailed information about the ceiling panels. Reproduced with permission of the NRCC from RP-755, January 1997.

If the existing rule-of thumb were to hold true, one would expect the heaviest weight (8.9 kg/m2
[1.8 psf]) gypsum board panels (type G13) to provide the most attenuation. In fact, it performed second to worst. The rigid material spans the high points in the supporting grid flanges, leaving gaps elsewhere through which noise leaks. Additionally, the gypsum board provides no absorption in the plenum or room. Even lower in attenuation performance is the very lightweight A2910 fibreglass panel. While it has the malleability to decrease leaks at the grid, it has low absorption (NRC 0.70) and very low weight (0.5 kg/m2 [0.1 psf]).

While stone wool ceiling panels were not offered in North America at the time of the study, their NRC rating, weight, and other attributes also falls within the range of the panels used in the study. Stone wool panels performed similarly to the thick, fibreglass panels (AHRI types 4 and 5). Both ceiling panels have the right combination of all three factors for noise attenuation. They have high NRC, moderate weight, and malleable surfaces that are soft and seal the leaks at the grid. While there are slight differences in the attenuation provided by the different ceiling panel types, those differences are not significant enough to affect building occupant well-being over time.

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